A particular application of infrared sources is in an infrared spectrophotometer, an instrument which measures the relative absorption spectra of chemical compounds. In such a system infrared radiation is collected optically from an infrared source and focused onto a cell containing a chemical sample. The sample characteristically absorbs selected wavelengths of the incident radiation. Transmitted wavelengths pass through the sample cell and are focused onto a photoelectric detector which produces an electrical output relative to the intensity of transmitted radiation. The sensitivity of a spectrophotometer is ultimately dependent upon the sensitivity of the detector, the intensity of radiation emitted by the source at each wavelength and the ability to efficiently focus the radiant energy through the sample cell and to the detector.
Several factors affect the desirability of an infrared spectrophotometer source. In addition to high infrared flux, it is highly desirable that the source be stable, reach equilibrium in a short period of time, and not thermally or chemically pollute the inner environment of the spectrophotometer. It is further desirable that the source have a life expectancy of at least 1000 hours at normal operating conditions, and when expired be easily and inexpensively replaced. Further, the source should be of relatively small size to provide optimum spacing between adjoining and optically coupled components in the spectrophotometer.
Commercial blackbody sources, although providing 99+% of the maximum Plankian infrared flux at all wavelengths for a given temperature and with optimum stability, have not been accepted as sources for spectrophotometers due to their excessive size and cost. These devices are standards for radiometry and photometry and consist of a variety of well insulated, precisely heated blackbody cavities, each integrally encased, as a stand alone instrument and not readily or inexpensively renewed.
In the prior infrared spectroscopy art, sources have been limited to a small number of incandescent non-gaseous elements: Nernst glowers, wire wound ceramic glowers, silicon carbide rods and metallic ribbon filaments. When heated, these elements emit radiation according to well known Plankian spectral distribution having intensity proportional to the absolute temperature and spectral emissivity of the radiating surface at each wavelength. These devices are small, inexpensive, readily replaced, and temperature equilibrate quickly. Unfortunately, the refractory materials which are used to construct these devices and are suitable for use at high temperatures have relatively poor spectral emissivities which usually decrease with increasing temperature, and, likewise, those materials with uniformly high spectral emissivities are limited to relatively low temperatures.
Nernst glowers are constructed from rod or tubes of refractory ceramics, usually zirconia and to a lesser extent yttria and thoria. Platinum leads located near the ends of each element conduct power through the ceramic, heating it to temperatures up to 2000 degrees K. Nernst glowers suffer from several short comings. First, they are not self-starting and require some means of auxillary heating to lower their high electrical resistance at room temperature. Second, as with most rod heaters, these elements are substantially power inefficient since the entire outer diameter of the rod radiates energy and usually only a small area on one outer segment of the rod is usable for focusing. The rods are also relatively long since they are supported and electrically connected through their ends. Third, these elements have relatively poor spectral emissivities, averaging about 0.75 over the spectrographically useful range of 2-20 micron wavelength, and as low as 0.15-0.30 at 3 micron.
Wire wound ceramic glowers overcome the self-starting deficiency of Nerst glowers. These elements are comprised of a ceramic rod externally wound or a ceramic tube internally wound with a precious metal wire, usually platinum or a platinum alloy, which serves to heat the ceramic. The windings are secured by a ceramic powder which is sintered to the base element. In each case the ceramic is the radiating surface, usually alumina or zirconia, and as such the wire wound ceramic glowers exhibit the same poor short wavelength characteristics as the Nernst glowers, and are effectively as power inefficient since they too are heated rods which radiate along the length of their outer diameters.
Perhaps the most commonly used infrared spectrophotometer source is the silicon carbide rod, commercialized by the Union Carbide Corp. under the tradename Globar. The Globar is a rod of bonded silicon carbide capped with metallic electrodes, usually silver, which serve to pass current through the Globar to heat it. The Globar is self-starting and has a spectral emittance which is relatively uniform and averages 0.89 from 2-15 micron wavelength, with only a narrow emittance loss to about 0.6 at 12 microns. The primary shortcoming of the Globar is its temperature limitation in air of about 1570 degrees K. It is also necessary to cool its end caps, usually with water, adding unwanted bulk and cost to the spectrophotometer. And, like other rod sources, the Globar is appreciably power inefficient.
Some recent interest has surfaced in the use of metallic ribbon filament sources. An alloy mixture of 80% nickel and 20% chromium has a relatively high spectral emissivity, approximately 0.91 over the range of 2-15 micron wavelength when oxidized and measured at 1400 degrees K. The nickel chromium alloy also has favorable electrical resistance for heating, is inexpensive, and easily replaced. The metallic ribbon filament is limited to temperatures of about 1400 degrees K., however, and has relatively poor longevity at this temperature in air, less than 1000 hours.
As is readily aparent, from the prior art, current spectrophotometer sources all suffer from undesirable restrictions on infrared flux, either temperature limitations or poor emissivity characteristics. The net effect on spectroscopy is less than optimum spectral sensitivity. These sources also impose undesirable thermal dissipation into their host instruments, usually requiring added expense and added instrument size associated with auxillary cooling and the need to physically isolate the source structure from the many temperature sensitive components in a spectrophotometer. Commercial blackbody sources, being laboratory instruments, have been incompatible with the size, cost and replaceability needs of spectrophotometer infrared sources.